The gain of a demodulator is prone to process and temperature variations. This results in gain variations, due to a deviation of the transconductance of transistors and the resistance of resistors used in such demodulators, from their target value.
In conventional systems, the DC transconductance of the transistors have been measured by a separate test structure (separate from the demodulator). This has two main disadvantages: (1) a mismatch between the transistors in the test structure and the transistors in the demodulator circuit leads to an inaccuracy in the calibration, and (2) the gain deviation due to the resistors cannot be calibrated.